1. Field of the Invention
The present invention relates to tomographic imaging. More specifically, the present invention relates to a system and method for extending the density range for tomographic imaging of potentially radioactive attenuating objects in an industrial environment.
2. Description of Related Art including information disclosed under 37 CFR 1.97 and 1.98
Radioactive waste contained in drums can be highly heterogeneous in matrix distribution and may also exhibit a non-uniform and unrelated distribution of radionuclides. Under such circumstances, accurate quantitative results can be difficult or even impossible to obtain using current tomography techniques.
Current automated production environment tomographic imaging devices produce a transmission image and an emission image of an object, such as radioactive waste drum. The transmission image is a voxel-by-voxel distribution of linear attenuation coefficients throughout the drum volume. The emission image is a voxel-by-voxel distribution of the source activity from within the drum. The transmission image data is then used to create an attenuation map which is further utilized in analyzing the emission image data and ultimately quantifying the radioactive contents of the drum.
The current techniques utilize a single transmission beam of a predetermined energy for obtaining the transmission map. This well suited for low to moderate density waste matrices of approximately 1.0 g/cc for a 55 gallon storage drum. Slightly higher densities may be achieved by using alternative approaches to the analyses. These include the uniform layer and the bulk density type analyses.
In the uniform layer approach, all the voxels in a given drum layer (or segment) are populated with the same average value of linear attenuation coefficient. In the bulk density approach all of the voxels in all the drum layers are populated with the same value of linear attenuation coefficient. Still, these analyses cannot go much beyond a density waste matrix of approximately 1.5 g/cc.
Current devices require at least 10 cps at the detector to obtain useful imaging data. If the drum being imaged were homogeneous in its matrix distribution, this would not be an issue. For a drum with a uniform density, the transmission beam could be chosen that would provide the necessary count rate to obtain useful data. However, if such a system were used to measure a drum with widely varying internal densities, the beam would likely be either too bright for areas with low density or too dim for areas with high density.
Currently, an operator must perform multiple assays when attempting to image a drum with widely varying density. First, the operator must image the drum with a low-intensity beam to prevent blinding of the detector in areas of low density. However, the beam will not penetrate the high density portions and will not produce useful data. Second, the operator must image the drum with a high-intensity beam to “punch-through” the dense regions. This will blind the detector in areas with low density and not provide useful data. Finally, the operator must combine the data in some useful way to recreate the distribution of the drum. This type of imaging is impractical and inefficient due to the time required to conduct a thorough assay. Also, the costs associated with purchase and maintenance of two transmission sources is significant.
Drums with uniform density distribution are almost never encountered in real-world imaging situations. Also, current systems can only handle low to moderate density waste matrices. Automated production environments are often required to process drums with much higher densities and sizes, making accurate assays difficult if not impossible. Accordingly, a need exists for a method and system that can extend the dynamic range of a tomographic imaging device to allow it to assay drums with a density of greater than 1.5 g/cc. Further, a need exists for a single device that can perform a thorough assay on drums from low to high density waste matrices, including drums that are too hot to handle.